At any given time, about 5 percent of the population of the United States suffer from major depression. It affects people of all ages, races and ethnic groups. For unknown reasons, women are almost twice as likely as men to suffer from depression. The lifetime prevalence of major depression is about 20 to 26 percent for women and 8 to 12 percent for men. Manic depressive illness is less common. Between 0.5 and 1 percent of the population suffer from this type of depression.

Depression can occur at any age, but the average age of onset is about 40. Although many people experience their first episode of depression in their late teens or early adulthood, the incidence of depression increases with age. The elderly are at a high risk of developing depression as they face multiple health problems or the loss of loved ones.

Unlike other diseases that an individual can contract only once in a lifetime, depression is a recurrent condition. Those who have had an episode of depression have better than a 50 percent chance of the depression recurring sometime in their lives.

Persons of any age or race may contemplate suicide as part of their depression, but older white men are more likely than younger individuals to actually commit suicide. Overall, about 15 percent of patients who have depression for more than one month commit suicide. Many of these patients seek medical help before their suicide, often within one month of their death.